If it is a regular pyramid you need to find out the base perimeter, multiply it by the height of the sides (when considered as triangles) and divide by two.
The height of the side can be found using Pythagoras's formula if you know the height of the pyramid and the length of a side. The side height of a pyramid is also known as the slanted height or L in a formula.
Formula:permiter of base x L(side height) ( P of B)(L)
p x L(slant)
I haven't done this in awhile